ODF for LCD production has been replacing conventional vacuum-capillary filling for the purpose of process time saving. ODF technology includes the steps of applying a sealant to a first electroded substrate to a pattern of a display frame using a dispenser, dropping liquid crystals inside the frame, attaching a second electroded substrate to the first substrate in vacuo, temporarily curing the sealant by UV irradiation, and heating the liquid crystals and the sealant to anneal the liquid crystals and to complete curing the sealant. According to the ODF process, curing the sealant in two stages, i.e., photocure and thermal cure, allows reduction of curing time, which leads to shortening the process time for the LCD manufacturing.
However, because the currently practiced conditions for sealant curing including UV irradiation and high temperature heating cause considerable damage to the liquid crystals treated concurrently, the demands, such as retention of electro-optical characteristics of liquid crystals, are not fulfilled. Additionally, there are demands for energy saving in thermal cure and further reduction of process time.
There is another problem with a conventional sealant. That is, when an uncured sealant comes into contact with a liquid crystal composition containing a compound having a carbon-carbon double bond, such as a liquid crystal compound and/or a polymerizable compound each having an alkenyl group, a (meth)acryl group, an allyl group, a maleimide group, etc. on its side chain, a photo radical initiator is eluted from the sealant into the liquid crystal composition. The eluted photo radical initiator can accelerate photo deterioration or induce unnecessary polymerization thereby to impair the reliability of the LCD.
Accordingly, a sealant from which a photo radical initiator is less elutable into a liquid crystal composition has been sought for.
As used herein, the term “polymerizable liquid crystal composition” denotes a liquid crystal composition containing a liquid crystal compound and/or a polymerizable compound each having a polymerizable functional group. Specifically, the term means a liquid crystal composition containing a liquid crystal compound and/or a polymerizable compound each having a carbon-carbon unsaturated double bond, at which two or more molecules thereof are capable of forming a covalent bond on reaction with a radical. More specifically, the term means a liquid crystal composition containing a liquid crystal compound and/or a polymerizable compound each having at least one of alkenyl, styryl, allyl, vinyloxy, (meth)acryl, maleimide, and so on bonded thereto conjugatedly or nonconjugatedly. The term “(meth)acryl” as used herein refers to acryl or methacryl.
Liquid crystal compounds having an alkenyl group having a small rotational viscosity coefficient are often used in TN, VA, IPS, or other mode nematic liquid crystal materials for TFT-LCDs for the purpose of raising response speed to address moving images and the like. As stated, however, when an uncured sealant contacts the liquid crystal material, a photo radical initiator is eluted from the sealant into the liquid crystal material to induce photo deterioration of the alkenyl group.
A display having liquid crystal alignment controlled by using a polymerizable liquid crystal composition is called a polymer stabilized liquid crystal display. A polymer stabilized liquid crystal display is produced by the steps of providing a liquid crystal cell containing a polymerizable liquid crystal composition in its uncured state and then photocuring the monomer in a desired alignment state with a voltage applied, whereby the alignment of the liquid crystals in the cell is controlled by the polymer network to achieve an increased response speed and a widened viewing angle. When a polymer stabilized liquid crystal display device is produced using a photocuring polymerizable liquid crystal composition by an ODF process, the steps for the production are as follows.
A sealant is patternwise applied to a substrate by means of, for example, a dispenser. A polymerizable liquid crystal composition is dropped on a substrate. Another substrate is attached thereto. Only the sealant is photocured by irradiation with visible light and then thermally cured. Thereafter, the polymerizable liquid crystal composition is UV cured. Since two photocuring steps are required as described, the cure selectivity between the sealant and the polymerizable liquid crystal composition is an important factor that influences the optical performance of the prepared device.
Patent document 1 (see below) discloses a combination of a high-molecular photo initiator having three or more aromatic rings and a resin composition containing 60 mol % or more of a photocuring functional group. The combination disclosed does not aim at curing making use of visible light.
Conventionally used latent epoxy curing agents include hydrazide curing agents and amine-adduct curing agents. A hydrazide curing agent, being a single compound, can be made less contaminating to liquid crystals by removing impurities through purification. However, curing with a hydrazide curing agent needs heating at 120° C. for about one hour, so that a curing agent which reacts at a lower temperature in a shorter time has been awaited. Of amine adduct curing agents those having a tertiary amino group are liable to form an amine salt to interfere with polymerization, which can result in a failure to obtain a cured resin with satisfactory physical properties. Those having a primary amino group tend to react with an epoxy resin and therefore have the disadvantage of short pot life. Patent document 2 (see below) proposes a solution to this problem by masking the primary amino group with an acidic substance and discloses a latent epoxy curing agent that enables cure at 80 to 100° C. However, the document neither describes nor suggests combining the curing agent with a photocuring resin for use as a sealant for ODF.
Citation List
Patent Document
    Patent document 1: JP 2007-3911A    Patent document 2: JP 2005-292801A